In Section 2, we formulate a solution of this problem on the basis of a
layer-by-layer mean-field calculation, which has been successfully used to
investigate the phase diagram of the ANNNI model [1]}. Although it
is simple to formulate and provides very sensible results, this mean-field
calculation demands a considerable numerical effort to unveil the fine details
of the phase diagram. We then restrict the analysis to the overall aspects of
the phase diagrams. In particular, we locate the paramagnetic transition
lines, which already indicate the presence of modulated structures.
